This invention relates generally to gas impermeable, thermoplastic packaging film which can be heat sealed to itself or to another material to form a flexible or semi-flexible package. The invention relates more particularly to the use of such film in packaging food products in which the packaged product is submerged in heated water or autoclaved or retorted for a substantially period of time for pasteurizing or cooking the packaged product, the packaging material being essentially non-degradable under such conditions.
There is a need in the food packaging industry for a packaging film from which packages can be made which are of improved structural soundness, and provide excellent toughness and package integrity in cook-in applications. The cooking conditions are often severe enough to cause delamination or degradation of the packaging material in some cases. For the sake of package appearance, it is desirable to have a precooked food product which is attractively packaged inside a preferably transparent film within which it is precooked.
The term "pasteurizable" as used herein is intended to refer to packaging material structurally capable of withstanding exposure to pasteurizing conditions while containing a food product. Many food products require pasteurization after they have been hermetically packaged to destroy harmful microbes which grow in the absence of air. Specific pasteurization requirements tend to vary by country; however, limiting conditions probably are submersion of the hermetically sealed product in water at 95.degree. C. for one hour. Thus, for a bag to be characterized as pasteurizable, structural integrity of the bag must be maintained during pasteurization, i.e. the bag must have superior high temperature seal strength and must be delamination resistant under such time-temperature conditions.
Optionally, the packaging material is heat shrinkable under pasteurizing conditions so as to provide an attractively packaged pasteurized food product.
The term "cook-in" as used herein is intended to refer to packaging material structurally capable of withstanding exposure to cook-in time-temperature conditions while containing a food product. Cook-in packaged foods are essentially prepackaged, precooked foods that go directly to the consumer in that configuration which may be consumed with or without warming. Cook-in time-temperature conditions typically refer to a long slow cook, for example submerging in water at 70.degree. -80.degree. C. for four to six hours. Such cook-in time-temperature requirements are representative of institutional cooking requirements. Submerging at 80.degree. C. for twelve hours probably represents the limiting case. Under such conditions, a packaging material properly characterized as cook-in will maintain seal integrity and will be delamination resistant. Optionally, the packaging film is heat shrinkable under these conditions so as to form a tightly fitting package and preferably should have some tendency for product adhesion to prevent cook out or collection of juices between the surface of the food product and the interior surface of the packaging material.
A number of characteristics are required for cook-in packaging film. First, packages made from such film must have seal integrity under such conditions i.e. the heat sealed seams should resist being pulled apart. As a corollary, the film sould be heat sealable either to itself or to another material. In the case of cook-in packages having thermoformed cavities for placement of a product, a non-forming cover web is typically vacuum sealed to the formed web to create the cook-in package.
Secondly, such materials must be delamination resistant, i.e. the multilayers making up the film must not separate or blister.
Additionally, the food contact layer of such film must qualify under the appropriate food laws and regulations for safe food contact.
A fourth consideration is the oxygen barrier or vapor barrier properties of the material, which are preferably high to maintain the freshness of the food contained therein.
Additional desirable characteristics in some package applications are heat shrinkability during the cook-in process to provide a packaging material which fits snugly around the product contained therein, as well as optical clarity where eye appeal of the packaged product is desired.
Various cook-in packaging materials are currently available in the market place U.S. Pat. No. 4,469,742 issued to Oberle et al describes a pasteurizable and/or cook-in shrink film which employs a hydrolyzed ethylene vinyl acetate copolymer (ethylene vinyl alcohol copolymer or EVOH) in a multilayer structure suitable for cook-in applications.
Another current film structure, such as shown in U.S. Pat. No 4,683,170 (Tse et al), has a five layer construction in which a core layer of ethylene vinyl alcohol copolymer is adhered to an inner ionomeric sealant layer, and to an outer nylon abuse layer, by means of respective tie layers, one on either side of the core layer. This latter structure requires the use of relatively thick tie layers. In some cases these tie materials, such as the Plexar series of resins available from Quantam and the CXA series of polymeric adhesive resins available fro du Pont, are typically very specialized and relatively espensive materials.
These tie or adhesive materials, in addition to their expense, typically lack the required toughness often required in the severe cook-in regime often encountered in food processing.
EVOH is well known for its moisture sensitivity. In the current structure just described it is necessary to extrude a relatively thick layer of the expensive adhesive layers in order to provide some moisture protection for the EVOH. That is, in order to prevent ingress of moisture both from the food product side (sealant side) as well as the outside environment (abuse layer side) only two layers are available on either side of the EVOH layer to provide such a moisture barrier.
It is therefore desirable to provide a more economical multilayer packaging material which takes advantage of the benefits of ethylene vinyl alcohol copolymer, but provides a more economical means of reducing moisture ingress into the packaging material during the cook-in process.
It is another object of the invention to accomplish the first object without sacrificing film toughness, and preferably actually increasing film toughness.
The present invention accomplishes both of these objects as described further below.